The quality stability of drugs in vials has been improved by laminating the leg portion of a rubber stopper, which is to be in contact with drugs, with an inert film, such as of fluororesin, to improve the chemical resistance of the rubber stopper. Known lamination methods include two-step molding processes (see FIGS. 1 and 2, and Patent Literature 1) and one-step molding processes (see FIG. 3).
Rubber stoppers produced by two-step molding processes are excellent in airtightness as the lower surface of their flange region and the base of their leg portion are provided as a rubber surface. The two-step molding processes, however, have a problem in that a molding step and a step of removing unnecessary parts both need to be performed twice, thereby resulting in poor productivity. Another problem is that oily preparations may reach the rubber surface to swell the rubber, thereby resulting in poor chemical resistance and airtightness.
On the other hand, one-step molding processes can produce rubber stoppers whose flange region has a lower surface entirely laminated with an inert film, and are also excellent in productivity as the number of steps is greatly smaller than that of the two-step molding processes. The airtightness of a vial can be achieved, for example, by bringing the inner wall and the upper surface of the vial into intimate pressure contact with the base of the leg portion and the lower surface of the flange region of a rubber stopper. However, disadvantageously, rubber stoppers produced by the one-step molding processes have poor airtightness because each of the above contact surfaces is hard glass or an inert film.